Wax-bearing lubricating oils are known to set to a semi-plastic mass on cooling below the temperature of the crystallization point of the wax contained in the lubricating oil. This change is measured in terms of pour point which may be defined as the temperature at which the oil sample is no longer considered to flow when subjected to the standardized schedule of quiescent cooling prescribed by ASTM D97-47. This problem presents a substantial disadvantage in the use of lubricating oils by the petroleum industry.
The problem with lubricating oils which contain any amount of waxes is that the wax contained in the oil, which is a paraffinic oil, will crystallize when the oil is cooled, and networks of wax crystals will then form on further cooling which will prevent the oil from flowing. The point at which the oil stops flowing is defined as the pour point temperature. Dewaxing of an oil improves the pour point, but this is an expensive procedure. Usually, the procedure is to dewax an oil to a certain temperature and then add pour point depressants to improve the low temperature properties. However, at the lower temperature, the same amount of wax will still separate. The pour point depressants do not make the wax more soluble in oil; they function rather by disrupting or preventing the formation of the waxy network As little as 0.2 wt. % of a good pour point depressant can lower the pour point of the paraffinic oil or lubricating composition by 30.degree.-35.degree. C.
The wax networks will also lead to an increase in oil viscosity. The increase in viscosity is generally temporary as a "normal" internal combustion engine can generate sufficient shear to disrupt the wax networks and allow the oil to flow. However, it should be emphasized that while the physical turning or cranking of the engine is usually unimpeded, the temporary disruption in the oil flow can lead to an increase in bearing wear.
Studies have indicated that the amount of wax needed to prevent flow or gel for an oil is quite small. Approximately 2% precipitated wax will gel middle distillates, and a similar amount is needed for lubricating oils.
Many different types of pour point depressants have been used in the prior art. Previously used pour point depressants are predominantly oligomers having molecular weights of 1,000 to 10,000, or polymers which have molecular weights greater than 10,000. The early point depressants were either alkylated aromatic polymers or comb polymers. Comb polymers characteristically have long alkyl chains attached to the backbone of the polymer, with the alkyl groups being of different carbon chain lengths.
The mechanism of action for pour point depressants has been the subject of much interest. Early indications were that alkylated aromatic compounds function as pour point depressants by coating the surface of the wax crystals and preventing further growth. More recently, however, it appears that the pour point depressants are either absorbed into the face of the wax crystal if the pour point depressant is an alkyl aromatic or co-crystallize with the wax crystal if it is comb polymer. Thus, crystal growth is not prohibited, it is simply directed or channeled along different routes. Light microscopy suggests that wax crystals are typically thin plates or blades, and when a pour point depressant is added to the system, those crystals are smaller and more branched, and thus the pour point depressant may disrupt or redirect crystal growth from different directions into a single direction and bulkier crystals will be formed. These crystals then can form networks only at much lower temperatures which results in a lower pour point.
Reports on pour points studies may be found in the publication by Gavlin et al entitled "Pour Point Depression of Lubricating Oils", Industrial and Engineering Chemistry, Vol. 45, 1953, pages 2327 to 2335. Also of interest in background with respect to pour point depressants is the publication by Clevenger et al, entitled "Low Temperature Rheology of Multigrade Engine Oils--Formulary Effects", 1983 Society of Automotive Engineers, Inc., Publication No. 831716; a publication by Henderson et al entitled "New Mini-Rotary Viscometer Temperature Profiles that Predict Engine Oil Pumpability", Society of Automotive Engineers, Inc. 1985, Document No. 850443; a publication by Lorensen, "Symposium on Polymers in Lubricating Oil Presented Before the Division of Petroleum: Chemistry, American Chemical Society, Atlantic City Meeting, Sept. 9-14, 1962, Preprint, Vol. 7, No. 4; and a publication by R. L. Stambaugh entitled "Low Temperature Pumpability of Engine Oils", Society of Automotive Engineers, Document No. 841388, 1984.
As pointed out above, the most recent interest in pour point depressants is found in poly(methacrylate) polymers. Indeed, methacrylate/acrylate polymers appear to be the most popular class of pour point depressants now in use. There is available commercially a line of poly(methacrylate) pour point depressants from the Rohm and Haas Company under the tradename Acryloid. Also available are similar products from Texaco under a trade designation of TLA followed by a numerical suffix or TC followed by a numerical suffix.
There has also been substantial patent activity concerned with pour point depressants which comprise poly(methacrylate) compositions. Thus U.S. Pat. Nos. 3,607,749 and 4,203,854 disclose poly(methacrylate) as viscosity index improvers, but without any data as to their low temperature performance. In particular, U.S. Pat. No. 3,607,749 discloses a blend of a high molecular weight polymethacrylate with a low molecular weight polymethacrylate as a viscosity index improver.
U.S. Pat. No. 3,598,736 discloses the addition of small amounts of oil soluble polymethacrylates to lubricating oils to reduce the pour point. The polyalkylmethacrylates are described as copolymers wherein the alkyl side chain contains from 10 to 20 carbon atoms with an average of between 13.8 and 14.8 carbon atoms. U.S. Pat. No. 3,679,644 is a division of U.S. Pat. No. 3,598,736 and contains the same disclosure.
U.S. Pat. No. 4,073,738 discloses the use of a pour point depressant which comprises an alkyl acrylate or alkyl methacrylate wherein the alkyl group side chain can have from 8 to 30 carbon atoms and preferably from 8 to 22 carbon atoms.
U.S. Pat. No. 4,088,589 discloses a combination of pour point depressants of which one can be an oil soluble polymer of an alkyl acrylate or methacrylate which contains a side chain comprising 10 to 18 carbon atoms in the alkyl group.
U.S. Pat. No. 2,655,479 of Munday et al is directed to polyester pour depressants and is particularly concerned with average side chain length of acrylate polymer pour depressants. The patent states in column 3, beginning at line 49 that polymers of single esters or homopolymers are not good pour point depressants but that copolymers are generally good pour point depressants. At column 4, beginning at line 44, it is stated that it is necessary that the average side chain length be in the range of about 11.0 to about 13.5 carbon atoms per mol of monomer. However, this patentee uses a combination of two polymers to obtain this side chain length and the results are unsatisfactory.
U.S. Pat. No. 3,598,737 discloses lubricant compositions which contain copolymers of acrylate esters which are said to improve various characteristics including pour point. This patent states that the average number of carbon atoms should be at least 12.5 to 14.3. These compounds do not appear to be acrylate esters wherein the side chain is this value, but rather this patent shows the use of hydroxyalkyl esters in a poly(methacrylate).
U.S. Pat. No. 3,897,353 discloses oil compositions comprising lubricating oil and a pour depressant which can be an alkylmethacrylate. These acrylates may be made from monomers wherein the alkyl portion of the ester or the side chain has from 12 to 18 carbon atoms and includes mixtures. However, the polymers of this patent are made from nitrogen-containing monomers.
The present invention, however, provides a pour point depressant based on poly(methacrylate) polymeric compositions which represent a narrow class of such compositions and which have advantageous properties in improving the low temperature properties of lubricating compositions while maintaining a good viscosity index.